The use of stereotactic frames and three-dimensional digitizers in so-called xe2x80x9cframelessxe2x80x9d stereotaxy is now well known and described in the literature. In the case of frame-based stereotaxy, typically a head ring is placed on the patient, usually his head, and an arc system is further attached to the head ring so as to direct a probe into the body quantitatively to achieve an internal target, such as a tumor. In a case of frameless stereotaxy, several types of three-dimensional digitizers, including mechanically articulated arms, optically coupled probes, electromagnetic coupling devices, ultrasonic and acoustic devices have been devised which give the operator a frameless and freehand means of identifying a target within the physical anatomy by relating the position of a navigator to a graphic representation of that anatomy derived from scanner image data. For example, a CT X-ray scan of the patient""s head may be done, and a set of two-dimensional scan slices may be collected and inputted into a computer graphic workstation. The workstation can then assemble these 2-D slices and render the anatomy in a three-dimensional representation, shown on the display means of the computer workstation, such as a cathode ray tube (CRT). By identifying at least three non-colinear points on the physical anatomy, as relates to the 3-D representation in the computer graphic workstation, one can register the entire data set in the computer workstation relative to the actual anatomy. By using a digitizer or navigator which has an encoding means to give position data of a navigator in real space back to the computer graphic workstation, and by touching a known point on a navigator to the three non-colinear points on the physical anatomy in a calibration maneuver, the digitizer may be calibrated relative to the anatomy and its position can be displayed on the computer graphic workstation display means as a virtual indication of the position and orientation of the digitizer. This is a standard technique in the field of so-called frameless stereotaxy.
There are several types of digitizers which can be used for this purpose, and they range from mechanically encoded arms, optically coupled devices, electromagnetically coupled devices, ultrasonically coupled devices, and other technologies. To this date, theses digitizers involved a probe or instrument means which is attached to the digitizer, so that when the probe tip is pointed to physical anatomy, the position of the tip on the three-dimensional graphics of the anatomy in the computer workstation can be represented. Various instrument holders can be applied to the digitizer to hold probes, suction tubes, forceps, and other instruments, and they may have a known calibrated length so as so make the mapping between physical and graphic space.
One of the objects of this invention is to enable the operator to use the digitizer to point at an internal target in the physical anatomy without having to attach a rigid or fixed probe means to the digitizer, so as to give a virtual presentation of where the digitizer would be pointing in the physical anatomy along a given probe line, without actually physically reaching that internal target point. It is notable that if one has a fixed probe of predetermined or calibrated length in the digitizer, and that probe is pointing along a line projected through a target, it is not immediately obvious by mechanical means what the distance between the probe tip and the desired anatomical target is. The probe tip may be placed, for example, on the external anatomy such as the scalp, pointing in the direction towards an internal target such as a tumor, and it would be desirable to know by an immediate physical observation what the distance from the scalp to the tumor is along the projected line of the probe. This could be calculated or visualized on the computer graphic workstation, but a physical visualization on the digitizer or probe means would be desirable. It is an object of the present invention to provide such mechanical means on the digitizer probe means.